I wanted to access the metaData so I could set the style based on the value for the summaryRenderer. This is something that you could do in the column renderer.
When adding a summary row renderer to a grid column. It only gives you the following parameters

val, params, data

Yet there is a fourth metaData option which is hidden in Architect. I went looking and eventually found it after a bit of debugging.

The metaData param is available on the renderer for a normal column, but is hidden on the summaryRenderer.
Is this meant to be hidden? Tried to track it back through the source to see if it's tagged private.

Even setting a ViewController method name does not give you access to add the param either.

note:
I've assumed it's called metaData, given it has the following signature (example):